Section 2: Differential Leveling

Overview

Differential leveling is the process of measuring vertical
distances from a known elevation point to determine elevations of
unknown points. The most common methods to determine elevation are through
the use of 1) a compensator type, automatic (engineering level)
and level rod(s), and 2) an electronic digital barcode leveling
instrument with barcode rod. A thorough knowledge of leveling principles
and proper application of methods and equipment will prevent costly
delays and generate the needed results and accuracy.

Preferred methods of obtaining elevations (in order of preference/accuracy)
are:

The method in Figure 4-3 uses the difference in elevation
between a known elevation and the height of the instrument, and
then the difference in elevation from the height of instrument to
an unknown elevation point.

Equipment

The Rod

Rods are, in essence, a tape supported vertically, and
used to measure the vertical distance (difference of elevation)
between a horizontal line of sight and a required point above or below
it. The most common types are the Philadelphia rod- a 2 piece rod
usually 13' in length, the Frisco rod- a 3 piece rod 12'-15' in
length, the Lenker rod- a moveable face, direct reading rod, and
the fiberglass-rod- a multi-section, extension rod from 8' - 45'
in length. Electronic digital levels use a barcode marked rod. Precision
leveling, known as First Order leveling, to extend or establish
vertical control over long distances, requires use of invar scale
rods.

All rods should be checked and maintained to ensure consistent
readings. Cleaning and adjustment or repair should be done as needed.

The Level

The compensator
type, automatic (engineer) level is made by various manufacturers,
and is a precision, self-leveling instrument, equipped with a built-in
compensator that automatically maintains a horizontal line of sight
and has a telescope with approximately 30-power magnification. The
level mounts on a standard surveying tripod, or a fixed-leg tripod
for more precise leveling.

The electronic digital level is also a precision, self-leveling
instrument with additional advantages. Advanced capabilities include
automatic measurement of height and distance by reading a barcode
rod, high precision by employing image processing technology, data display
and data recording either internally or to a data collector, installed
software for elevation stake-out or other leveling procedures. The
digital level offers greater productivity and simplicity in all
applications of leveling work.

All leveling equipment must be checked regularly and properly
maintained to ensure that it remains in proper working condition
and that systematic errors are eliminated to produce the expected
results. This includes daily checks, periodic routine maintenance
and yearly cleaning and adjustment by a qualified repair shop.

Daily - keep the instrument clean by wiping occasionally,
especially when used in dusty or wet environment. Operate and adjust
the motions. With a compensator type, automatic level, observe the
compensator while adjusting level screws to make sure that motion
is smooth. If erratic compensator movement is detected, have a qualified
repair shop service the instrument.

Datum

Three-Wire Leveling

Three-wire leveling is the process of reading and recording
a rod reading for each of the three horizontal cross-hairs on each
shot, then averaging the readings for agreement with the center
reading. This method is most accurate as it gives immediate results
and a check/confirmation of rod reading. If a difference is detected
a check can be done on the spot, before moving on.

An electronic digital level can perform and record this process
automatically, after sight of the rod is made. Automated data collection
eliminates transposition errors and data is downloaded into a computer
for complete analysis.

This is the preferred method for setting or checking control
bench marks. Three-wire leveling will be used for all orders of
leveling except general.

Bench Marks (BM) and Temporary Bench Marks (TBM)

The primary purpose of running a level loop is to establish
points of known elevation or bench marks. They are solid, well protected
points that can be relied upon to remain undisturbed and unchanged.
They should be positioned so as to be usable from a wide area and
away from the construction as much a possible so as to remain undisturbed.
Consideration should be given to construction activities such as
utility re-location proposed within the ROW. It is preferred that bench
marks are located in a public ROW, to allow for continued accessibility.

The exact location of the bench mark should be carefully selected,
then sketched and recorded in notes, a field book or a data collector.

The numbering or identification system should be consistent
with district numbering convention, if any and should be identified
with enough detail for another crew to locate the bench mark easily. Bench
marks set should always be turned through as part of a closed level
loop.

Examples of good bench marks are an iron rod driven
to refusal or a TxDOT cap set vertically in a concrete monument
(similar to a Type II ROW Monument), a ROW monument, a point or
corner of a stable concrete structure, or occasionally a spike set
in a large tree or utility pole. It is usually advisable to use
a variety of types of bench mark monuments as utility relocation
or construction activity may remove objects from a ROW.

The bench marks should not include objects subject to tampering
or removal.

Two or more bench marks should be used from the specified
reference datum (NAVD 88 unless directed by TxDOT). These bench
marks should be identified, confirmed undisturbed, and elevations
proved by running a complete level loop between the two, returning
to the starting bench mark.

Distance between bench marks should be confirmed with the
TxDOT project manager or the district survey coordinator before
beginning a project.

Turning Points/Temporary Bench Marks (TP/TBM)

Turning points may be points set either before or during the
course of the survey, or natural or man-made points in the area.
They must be solid, well defined (or marked) and permanent enough
to remain intact until the level loop is finished. Points with a
small, sharply defined top are preferred to large flat surfaces.

Turning points should be marked when used so as to insure
that the rod is in the exact same place when the backsight and foresight
are taken. They are also marked because turning points that are part
of a closed level loop are points of known elevation that may have
value during future surveys in the area.

Temporary bench marks set in trees or power poles should consist
of a spike (railroad spike, boat spike, or large nail spike) set
horizontally approximately 1 to 2 ft. above ground elevation, also
free of above obstructions to the level rod.

General Considerations / Objectives

During the course of running a level loop, choose/set turning
points and bench marks to accomplish the required objective and
accuracy.

Balance
shot distances – The rodman and instrument man must work
as a team to balance the backsight and foresight distances. This
can be accomplished by use of a digital level, estimating distance
by pacing, three-wire stadia difference or, when available, by observing
stationing marked on the project. Balanced backsights and foresights,
essential in precise leveling, will help eliminate errors caused
by refraction, the curvature of the earth and an instrument that
is out of adjustment and are an especially necessary procedure when
establishing control bench marks.

Maximum
sight distance – care should always be given to observe
recommended or required distance of sight, depending on the purpose
of leveling. See the
NGS specifications
and
Table
4.3 TSPS Manual of Practice Chart for Tolerances for Conditions,
found in this chapter, or the manufacturer’s recommendations.

Control points and bench marks should always be set to the
highest level of accuracy suitable for the project or a higher level
if it can be justified.